The present invention relates to an intake manifold that has branch pipes connected to the intake ports of the cylinder head of an internal combustion engine.
Some of such intake manifolds include valves each provided at the downstream end of a branch pipe to change the cross-sectional area of the intake passage (refer to Japanese Laid-Open Patent Publication No. 2009-2218). The intake manifold of the document changes the opening degree of the valves to change the cross-sectional area of each intake passage, thereby generating tumble flows in the combustion chambers. Such intake manifolds are often made of heat resistant plastic.
During operation of an engine, the cylinder head is heated to high temperatures and the heat is transferred to the intake manifold from the cylinder head, which thermally expands the branch pipes. In an intake manifold having valves at the downstream ends of the branch pipes to change the cross-sectional area of each intake passage, thermal expansion of each branch pipe may cause its inner wall to interfere with the valve. Conventionally, the clearance between the branch pipe inner wall and the valve is designed to be large with some margin to avoid interference between the inner wall and the valve. However, such a clearance tends to be unnecessarily large even when the branch pipe is relatively cold and not significantly expanded due to heat. In this case, even when the valve is closed, the clearance is likely to cause leakage of intake air therethrough. It is thus difficult to change the intake air flow in a favorable manner, and desired tumble flows may fail to be created.